The Human Ether-a-go-go Related Gene (HERG, KCNH2) encodes the major, pore-forming subunit of the cardiac K+ current IKr. Suppression of IKr, through inherited mutations or pharmacologic blockade, can provoke sudden death from a ventricular arrhythmia (Torsades de Pointes). Like IKr, HERG channels are sensitive to a wide array of therapeutic agents but in practice, the development of cardiac arrhythmias upon exposure to HERG-blocking compounds is unpredictable, suggesting modulating factors critically influence HERG pharmacology. The goal of this proposal is to identify molecular mechanisms that mediate drug interactions with the IKr complex. While HERG block by most pharmacologic agents develops as the membrane is depolarized and channels open, block still develops slowly (over minutes) suggesting that access of drug to its receptor site in the inner pore vestibule (S6) is limited. While the mechanisms that underlie drug interactions with HERG are incompletely understood, our recent studies have identified the HERG C-terminus and a HERGinteracting protein (KCR1) as inhibitors of block. We will test the hypothesis that HERG blockade by therapeutic compounds is modulated by functional interactions involving HERG subdomains and other proteins that compose the IKr complex. Using electrophysiologic and biochemical approaches, Cterminal deletion mutants, and C-terminal peptides, we will determine the mechanism whereby the HERG C-terminus limits drug access to the pore. Using the same approaches, we will elucidate the molecular mechanism whereby human KCR1 inhibits drug block. Finally, to expand our understanding of the IKr complex and the molecular substrates of proarrhythmic risk, we will utilize the enetically tractable organism C. elegans as a model system to identify new HERG-interacting protein candidates, taking advantage of the association among the C. elegans homologue of HERG (UNC-103), methanesulfonanilde drug action, and the rhythmic pattern of pharyngeal pumping in the worm. The improved understanding of drug-channel interactions arising from this research should enable improvements in predicting risk for drug-induced arrhythmias, and the development of improved antiarrhythmic therapies.